Radio signaling



June 23, 1942. P. 's. CARTER RADIO SIGNALING Filed Feb. 28, 11939 TRANSMITTER COMPENSATING (INDUCED RAY INDIRECT RAY;

REFLEC TING SURFACE C OMPENS A TING REFLEC'T/NG 5 URFA C E RECEIVER v 2 PEFLECT/NG COMPENSAT/A/G REFLECTOR v OBJECT TEAp/SM/TTER I INVENTOR. P/IZP .9. CARTER sou/20E 0F UNDES/RED SIGNALS REC El VER ATTORNEY.

Patented June 23, 1942 RADIO SIGNALING Philip S. Carter, Rocky Point, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Claims.

This invention relates to short wave radio transmission and, more particularly, to a means for eliminating interference due to multi-path transmission caused by indirect rays from a transmitter being reflected from buildings and the like and being picked up at the receiver in addition to the direct rays.

An object of the prevent invention is to overcome interference due to multi-path transmission between a transmitter and a receiver.

Another object of the present invention is to cancel interference due to multi-path transmission by providing an additional path for the transmitted signal having a length differing by an odd multiple of the operating wave length from the path taken by the interfering signal.

Still another object of the invention is to eliminate undesired signals arriving at a receiver by combining with the said signal another similar signal in a phase opposing relationship whereby the undesired signals cancel.

Briefly, my invention involves setting up a reflection which will balance out or cancel an undesired other reflection at a receiver. Assuming that the direction of a signal is known and that there is received at the receiving station an undesired reflection caused by some object, such as a water tower, at building, a smoke stack of some sort, etc., between the transmitting station and the receiving station but located somewhat off the direct line of travel in accordance with the invention, I propose to set up an extra reflector mounted either on the first reflecting object or located some distance away so as to cause another reflected ray to cancel the first undesired reflected ray. The difference in the paths of the two reflected rays caused by the reflected object and the extra reflector is arranged to be equal to 180 degrees, or some odd multiple thereof. The intensity of the two rays or beams at the receiving apparatus should, of course, be substantially equal in order to attain satisfactory cancellation.

Referring, now, for a more complete understanding of the invention to the following detailed description, which is accompanied by a drawing, in which Figure 1 illustrates one form of my invention, while Figure 2 illustrates a modification of my invention for eliminating interfering signals from a second transmitter and Figure 3 illustrates a form of reflector which is particularly adapted for use with comparatively long waves.

In Figure 1, reference numeral l indicates a transmitter such as a television transmitter or similar short wave transmitter. Numeral 2 indicates a receiver which receives by direct ray 3 the undesired intelligence. Due to reflecting object 4 an indirect ray 5 is also received at the receiver 2 from transmitter I. This indirect ray is considered in this embodiment to have a total length I. In order to compensate for this indirect ray in accordance with my invention, I place a compensating reflecting surface 6 at such a point that the compensating indirect ray I has a total length of path equal to I, plus or minus an odd multiple including unity of a half of the length of the operating wave. Thus it will be seen that the two indirect rays 5 and I arrive at the receiver in substantially phase opposition and cancel, leaving only the direct ray 3 to actuate the receiver.

In some cases the undesired signals arriving at the receiver may not originate from the same transmitter as the desired signals. This state of affairs is illustrated in Figure 2 wherein signals from transmitter I are transmitted by direct path 3 to receiver 2. At the same time from a source of undesired signals I0 signals are radiated which strike reflecting object I l and are reflected back to receiver 2 along path I5. The arrival of these signals at the receiver 2 causes interference with the signals transmitted from the transmitter I by direct path 3. In order to compensate for this interference a compensating reflector I6 is so placed that a second ray of signals is transmitted from source I!) to receiver 2 by path IT, for example. Path I1 is, as described with reference to path I in Figure 1, equal in length to the length of path I5 plus or minus an odd multiple of the half wave length of the signals from source III. Thus the signals arriving at receiver 2 by paths I5 and Il cancel and the interference is eliminated.

In the foregoing description it has been assumed that compensating reflectors 6 and I6 are sheet reflectors which may be made'with overlapping segments so as to be adjustable in area. to obtain a maximum efficiency. For longer waves or where a sheet reflector may not be practical, a simple dipole antenna which will receive the wave and re-radiate the same so as to provide another wave to cancel the first reflected area may be provided. In other words, the sheet reflector may be replaced by an antenna which may be called a neutralizing or parasitic re-radiating antenna. Such an antenna is shown diagrammatically in Figure 3 and may comprise two collinear quarter wave. sections I8 and I9. This antenna is located in the same way as previously disclosed for the sheet type reflector in the previously explained modifications. The strength of the re-radiated wave may be adjusted by varying the variable resistor l9 connected between the adjacent arms of the dipole. Any necessary phase adjustment may in all cases be obtained by changing the position of the compensating reflector as desired, whether it be a sheet reflector or a dipole reflector. When using a dipole reflector, as shown in Figure 3, the polarization of the neutralizing reflecting ray may be changed by properly orientating the dipole about an axis perpendicular to the axis of conductors l8 and I9.

While I have particularly shown and described several modifications of my invention it is distinctly understood that my invention is not limited thereto but may be Varied within the scope of the invention.

I claim:

1. In a signaling system having a transmitter and a receiver with an antenna connected thereto, there being more than one path for intelligence modulated radiant energy from said transmitter to said receiving antenna, the method of avoiding interference caused by said multiple paths which comprises combining at said receiving antenna the energy arriving on all except one of said paths with similar energy in an opposing phase relationship whereby the effect of said undesired radiant energy paths is neutralized and receiving the intelligence modulated radiant energy arriving on said remaining path.

2. In a signaling system wherein intelligence modulated signals from a transmitter arrive at a receiving antenna by a direct path of one length and interfering signals by an indirect path of a different length, the method of compensating for said interfering signals which comprises providing an additional path for said signals from said transmitter to said receiving antenna, said path differing in length from the length of one of said other paths by 180 electrical degrees, combining the signals arriving by said additional path and one of said other paths at said receiving antenna, and receiving the signals arriving by the remaining one of said paths.

3. In a signaling system wherein intelligence modulated signals from a transmitter arrive at a receiving antenna by a direct path of one length and interfering signals by an indirect path of a different length, the method of compensating for said interfering signals which comprises providing an additional path for said signals from said transmitter to said receiving antenna, said path differing in length from the length of one of said other paths by 180 electrical degrees, combining the signals arriving by said additional path and one of said other paths at said receiving antenna, and receiving the signals arriving by said direct path.

4. In combination, a transmitter and a receiver, a reflecting object in the field of said transmitter whereby interfering signals arrive at said receiver by an indirect path and means for compensating for said interfering signals comprising a second reflector within the field of said transmitter and so arranged that signals are reflected to said receiver by a second indirect path, the length of said second indirect path differing by an odd multiple of half the interfering wave from the length of said first indirect path whereby the interfering signals are neutralized.

5. In combination, a transmitter and a receiver, means for transmitting signals from said transmitter to said receiver by a desired path, a reflecting object within the field of said transmitter whereby interfering signals arrive at said receiver by an indirect path and means for compensating for said interfering signals comprising a second reflector within the field of said transmitter and so arranged that signals are reflected to said receiver by a second indirect path, the length of said second indirect path differing by an odd multiple of half of the interfering signal wave length from the length of said first indirect path whereby the interfering signals are neutralized.

6. In combination, a transmitter and a receiver, means for transmitting signals from said transmitter to said receiver, a second transmitter, a reflecting object within the field of said second transmitter whereby interfering signals from said second transmitter arrive at said receiver, and means for neutralizing said interfering signals comprising a second reflector within the fleld of said second transmitter and so arranged that the length of the path of signals from said second transmitter to the receiver by way of said second reflector differs from the length of the path of said interfering signals from said second transmitter to said receiver by way of said first reflector by a half of the length of the wave of said interfering signals.

'7. In combination, a transmitter and a receiver, means for transmitting signals from said transmitter to said receiver, a second transmitter, a reflecting object within the field of said second transmitter whereby interfering signals from said second transmitter arrive at said receiver, and means for neutralizing said interfering signals comprising a second reflector within the field of said second transmitter and so arranged that the length of the path of signals from said second transmitter to the receiver by way of said second reflector differs from the length of the path of said interfering signals from said second transmitter to said receiver by way of said first reflector by an odd multiple of half of the wave length of said interfering signals.

8. In a signaling system wherein intelligence modulated signals from a transmitter arrive at a receiving antenna by a direct path of one length and interfering signals by an indirect path of a different length, the method of compensating for said interfering signals which comprises providing an additional path for said signals from said transmitter to said receiving antenna, said path difiering in length from the length of said indirect path by electrical degrees, combining the signals arriving by said additional path and said indirect path at said receiving antenna and receiving the intelligence modulated signals arriving by said direct path.

9. In a signaling system wherein intelligence modulated signals from a transmitter arrive at a receiving antenna by a direct path of one length and said signals also arrive at said receiving antenna by an indirect path of a different length, the method of compensating for interference between said signals which comprises providing an additional path for said signals from said transmitter to said receiving path, said path differing in length from the length of one of said other paths by an odd multiple of a half of the wavelength of said signals, combining the signals arriving by said additional path with the signals arriving by one of said other paths at said receiving antenna and receiving the signals arriving by the remaining one of said paths.

10. In a signaling system having a transmitter and a receiver with an antenna connected thereto, there being one direct path for intelligence modulated radiant energy from said transmitter to said receiving antenna and at least one indirect path, the method of avoiding interference caused by said plurality of paths which comprises combining at said receiving antenna the energy arriving on said indirect path or paths with similar energy in an opposing phase relationship whereby the efiect of said indirect path or paths is neutralized and receiving the intelligence modulated radiant energy arriving on said direct path.

PHILIP S. CARTER. 

